1. Field of the Invention
The subject disclosure relates to a control system for use with aircraft gas turbine engines, and more particularly to, a control system that includes a real-time vibration monitoring system having a diagnostic and a prognostic component.
2. Background of the Related Art
The gas turbine engine includes several concentrically mounted components, such as shafts, bearings and gears, each rotating at a slightly different and known frequency. Machinery having massive rotating components, such as jet aircraft engines, may experience shaft, bearing, and/or gear failures. In addition, these rotating components may become unbalanced and impose loads upon the bearings and engine housing well beyond acceptable specifications. These problems may be a result of any variety of causes which include, manufacturing defects, design defects, wear, misuse, accidental damages and the like. In the case of in-flight aircraft, failure of these engine components can lead to, not only engine loss, but a catastrophic loss of aircraft and crew.
Vibration amplitudes and patterns, induced by the rotation of jet engine sub-components, can be indicative of sub-component degradation and decreased reliability. Vibration detection is not only an invaluable safety tool used to monitor engines while in operation, but has been incorporated into scheduled engine maintenance procedures. Various techniques have evolved in the art to detect and analyze engine sub-component vibration amplitudes and patterns. For example, U.S. Pat. No. 4,488,240 to Kapadia et al. discloses a vibration monitoring system for gas turbine engines, which includes a non-recursive digital filter network controlled by a data processor. Digital techniques are used to process tachometer signals and general sampling signals at frequencies which are an integral multiple of the component rotating frequency. The output of the digital filter network is a signal used to drive a display device.
Systems such as that disclosed in the Kapadia et al. patent include various hardware and circuitry, for example, micro processors, multiplexers, analog signal conditioning circuitry, analog to digital signal converters, digital filters, digital to analog signal converter of output and a variety of memory chips.
Typically, a vibration analysis system consists of an analysis module which is connected to a generic vibration sensor located at the engine. In addition, the analysis module must access engine specific data for comparison and analysis. The analysis module may be integrally installed in an aircraft for monitoring vibrations during operation. The analysis module may be located in the engine housing or located somewhere else in the aircraft. The design considerations of the location of the analysis module may take into account optimal vibration sampling considerations and space constraints.
A disadvantage of prior art vibration monitoring systems is that they do not include a diagnostic component which is capable of comparing in real-time the measured vibration amplitudes for various shafts, bearings and gears to stored engine specific data. Additionally, prior art systems do not include a prognostic function wherein the time remaining to component failure is predicted based on the measure vibration levels for that component and stored engine specific data.
There is a need therefore, for a control system which includes a vibration monitoring system that has a diagnostic and a prognostic component and is capable of evaluating, in real-time, measured changes in the vibration amplitudes for multiple components.
The disclosure of the present application relates to a vibration monitoring system for use with a gas turbine engine that includes xe2x80x9creal-timexe2x80x9d diagnostic and prognostic components. The vibration monitoring system disclosed herein represents one possible configuration of a system which acquires vibration data from an engine, and processes the data with advanced algorithms to determine engine component health, both in a diagnostic and prognostic fashion. Those skilled in the art to which the application appertains would readily appreciate that the system disclosed herein can be used on machinery as well as other types of mechanisms that include rotating components.
The present disclosure is directed to a method for monitoring the vibration levels associated with a rotating component and establishing an alarm setting therefor. The method includes measuring an operating parameter and a corresponding set of vibration amplitudes for a rotating component during a period of operation and normalizing the set of measured vibration amplitudes based on established amplitude limits for the rotating component so as to define a set of normalized amplitude data points. The established amplitude limits are a function of the measured operating parameter for the component. In a preferred embodiment, the measured operating parameter is a frequency of rotation for the component. Alternatively, the measure operating parameter is a rotational torque for the component.
The method further includes storing the set of normalized amplitude data points into parameter-based data blocks, each data block extending over a predetermined range of the operating parameter. For each data block, a time period remaining to reach the established amplitude limits is estimated based on changes in the normalized amplitude data points stored in the data blocks. Based on the estimated time period remaining to reach the established amplitude limits for each data block an alarm setting is established.
In a preferred embodiment, the method further includes ensuring that the measured operating parameter for the component is approximately constant (i.e., steady-state) over a predetermined data collection period prior to measuring the corresponding set of vibration amplitudes. It is presently envisioned that the step of measuring a corresponding set of vibration amplitudes includes conditioning measured vibration accelerations for the component using a Fast Fourier Transform. Those skilled in the art would readily appreciate that other transforms can be used convert data from the time to the frequency domain.
Preferably, the method disclosed herein further includes the step of providing an alarm signal to the operator based on the alarm setting if at least one of the measured vibration amplitudes exceeds an established amplitude limit.
In a representative embodiment, the set of normalized amplitude data points a stored in parameter-based data blocks which extend over a range of about 3% of a rated speed for the component and the data blocks have a spacing of about 1% of the rated speed. Additionally, the set of normalized amplitude data points can be stored in parameter-based data blocks which extend over a range of about 10% of a rated torque for the component and the data blocks have a spacing of about 5% of the rated torque.
Preferably, the normalized set of amplitude data points stored in each of the parameter-based data blocks are interpolated so as to estimate the time remaining to reach the established amplitude limits.
The present disclosure is also directed to a method for monitoring vibration amplitudes associated with a plurality of rotating components and establishing an alarm setting for each component. The method includes measuring an operating parameter and a corresponding set of vibration amplitudes for each of a plurality of rotating components during a period of operation. A rotating component to be monitored is selected from the plurality of rotating components and the corresponding set of vibration amplitudes are conditioned so as to eliminate vibration amplitudes associated with the unselected components in the plurality of components, creating a set of remaining vibration amplitudes.
The set of remaining vibration amplitudes are normalized based on established amplitude limits for the selected component so as to create a set of normalized amplitude data points. The established amplitude limits are a function of the measured operating parameter for the selected rotating component. The set of normalized amplitude data points for each component are stored into associated sets of parameter-based data blocks, each data block extends over a predetermined range of the measured operating parameter.
Still further, the method includes the step of estimating, for each data block, a time period remaining to reach the established amplitude limits based on changes in the normalized amplitude data points stored in the sets of parameter-based data blocks over the period of operation. An alarm setting for the selected component is then established based on the estimated time period remaining to reach the established amplitude limits for each of the data blocks. The method is repeated for each of the plurality of components until an alarm setting is established for each.
The present disclosure is also directed to a system for monitoring vibration levels associated with a plurality of rotating components and establishing an alarm setting for each component. The system includes mechanisms for measuring an operating parameter and a corresponding set of vibration amplitudes for each of a plurality of rotating components during a period of operation. The system further includes a mechanism for selecting from the plurality of rotating components a component to be monitored and a device for conditioning the set of vibration amplitudes so as to eliminate vibration amplitudes corresponding to unselected components in the plurality of components, thereby creating a set of remaining vibration amplitudes. The selected component can be for example, an engine shaft, bearing or gear.
Still further the system includes a mechanism for normalizing the set of remaining vibration amplitudes based on established amplitude limits for the selected component so as to create a set of normalized amplitude data points. The established amplitude limits are a function of the measured operating parameter for the selected rotating component.
In a representative embodiment, a mechanism for storing the set of normalized amplitude data points for each rotating component into associated sets of parameter-based data blocks is included. Additionally, the system includes a device for estimating, for each data block, a time period remaining to reach the established amplitude limits based on changes in the normalized amplitude data point stored in the data blocks over the period of operation and a mechanism for establishing an alarm setting for the selected component based on the estimate time period remaining to reach the established amplitude limits for each of data block.
In a preferred embodiment, the mechanism for measuring a set of vibration amplitudes for a plurality of rotating components during a period of operation includes at least one vibration sensor. In a representative embodiment, at least one sensor is a speed sensor for detecting and signaling frequency of rotation for one of the plurality of rotating components.
It is envisioned that the mechanism for measuring a corresponding set of vibration amplitudes can include means for conditioning measured vibration accelerations for each component using a Fast Fourier Transform.
Preferably, the measured operating parameter is a frequency of rotation for the selected component. Alternatively, the measured operating parameter can be a rotational torque for the selected component.
In a representative system, small accelerometers are used to sense vibration levels or amplitudes and the performance characteristics of these sensors are specifically defined and selected to work in symbiotic fashion with the interface filtering and electronics. Additionally, special signal processing or filtering/conditioning has been designed to match the performance of the accelerometers. This processing extends the frequency range of the accelerometers beyond that normally achieved by similar sensors with conventional filtering. A special combination of filters is used to provide anti-aliasing filtration without degradation of bandwidth.
In the method and system disclosed herein, amplitude trending is used to track growing vibration levels at specific frequencies, and at specific operating conditions. Warning and alarm levels are used to provide thresholds when the pilot is notified of a problem or pending problem. The vibration amplitudes are normalized by dividing them by the alarm level and the resulting data is stored into specific records as a function of speed and operating conditions. Additionally, a method is employed which records vibration amplitudes synchronously with speed data to allow specific extractions of relevant waveforms for use in developing diagnostics and prognostics targeted at individual rotating components.
Special techniques are used to extract bearing tones associated with failure or wear from higher frequency structural resonances. The high frequencies are demodulation and converted to lower frequencies, where the fundamental frequencies are then removed. The remaining tones (residuals) are then examined for signs of growing amplitude over time, an indication of wear.